My game is a Flash to HTML5 port, it’s nominal size is 760x630px, and even without additional WebGL optimizations (like Tilemaps) it shows 60 fps in most browsers even on my 6 years old laptop.
But… I’ve changed the game
<div> to occupy 100% of the browser window, so at 1920x1080 screen resolution the stage size is now around 1920x916px: everything around the main game sprite is either a solid or tiled background.
And now this is a problem even for my gaming PC, which can hardly support a plank of 40 fps!
As I explain it, the slowdown is due to all background pixels being redrawn on every frame. Unless I’m wrong, of course, what secret optimizations can be conducted to prevent this static graphics from consuming so much rendering time?